Uses for the Internet and the World Wide Web are continually increasing, and have expanded into “secure” areas. Different mechanisms for maintaining security in a network such as the Internet have been developed, such as the Secure Sockets Layer (SSL) security protocol. The SSL protocol uses a public key infrastructure to maintain security. In establishing an SSL connection between a client computer and a server computer hosting a web page, the server computer transmits a certificate to the client computer for verification or validation.
Typically in practice, when a user's Web browser first tries to contact a server for a secure transaction, the server sends its digital certificate to the browser. This certificate includes (among other things) the server's public key, the server's identity, the name of the certificate authority (CA) that signed the certificate and the signature itself (which is a mathematical hash of the certificate encrypted with the CA's private key). To validate the certificate, the browser computes the certificate hash and compares the result with the hash obtained by decrypting the signature using the CA's public key (as well as checking the validity dates and identity included in the certificate against the desired server). To then validate the server, the browser encrypts a message with the public key obtained from the certificate and sends it to the server. If the server can prove it can decrypt that message then it must have the associated private key and the authentication has succeeded. If desired, the server may likewise validate the browser. Once the browser and (optionally) the server is/are satisfied that each is the computer it claims to be, the browser and server can exchange session keys (additional keys that are used to encrypt the data transfers between the computers from then on).
In web-based systems, such as electronic commerce systems, when data is requested by a client from a server, it is often the case that the web server must query a database to locate the requested data.
In such a case, communications between a server and a web browser client typically require authorization of the client, to permit a client access only to certain data stored by the server. Such data may include, for example, contract information or pricing information which is exclusive to that client; other clients of the web server are not entitled to view this information.
One approach to identifying the client to the server is to initially authenticate the client and to then provide a session identifier to the client in the form of a hypertext transfer protocol (HTTP) cookie. A cookie, which is a form of persistent state object, is a small piece of data generated by the server and which is intended to be passed by the client with every subsequent client request to any server in a defined domain. Upon receipt of the request, the server can verify the client's entitlement to the requested information by comparing the contents of the cookie to the client records stored on the server. Such an approach is disclosed, for example, in U.S. Pat. No. 5,875,296 to Shi et al. (Feb. 23, 1999) in which a cookie including a client identifier is used to access an in-memory credential database used to allow or disallow access to files on a distributed file system. Browser uniqueness is also the subject of “How unique is your web browser” by Electronic Frontier Foundation at http://panopticlick.eff.org/browser-uniqueness.pdf.
Data and cookies that are transmitted between servers and clients on the Internet are subject to certain security risks unless measures are taken to secure communications between the client and server. An unauthorized user at a routing point or at another server in a cookie's domain may capture the packets transmitted between the client and the server and read the data contained in the transmitted cookie. Alternatively, a user may edit the contents of his or her own authorized cookie and alter the session data contained therein to construct a fraudulent session. For example, if the session data includes a contract identification number, the user could edit the cookie to insert a different number and thereby gain access to unauthorized data or resources when the edited cookie is transmitted to the server in a subsequent request. An unauthorized user may furthermore “steal” a valid cookie from an authorized user, and use the cookie to replay a valid session, thereby gaining unauthorized access to the server (a “replay attack”).
Further, as the size and diversity of the Internet grows, so do the devices and applications that use the network. Originally, network applications such as web browsers, terminal clients, and e-mail readers were the only programs accessing the Internet. Now, almost every new device or application has a networking component, whether it is to obtain content, updates, manage licensing, or report usage statistics.
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